The DDRAM or (Display data RAM) contains the character data that is displayed on the screen. It stores the 8-bit standard characters preset for the display in 80x8 bits for up to 80 characters. Their address locations can be found on page 5 of the display specification: http://www.newhavendisplay.com/specs/NHD-0216K1Z-FSW-FBW-L.pdf

The CGRAM or (Character Generator RAM) is customizable, and can be used to write custom character patterns in a 5x8 dot format. Users can program this area to create and display up to 8 custom characters.Details on how to program the CGRAM can be found on page 16 of the controller specification: http://www.newhavendisplay.com/app_notes/ST7066U.pdf